[TowerTalk] Rotator Choice for Larger Yagi

[Chuck Dietz]

This was on Eham.net:

In addition to windload and torque ratings, Yaesu also offers a
specification called "K-factor," which is derrived by multiplying the
turning radius of the antenna (in feet) by the weight of the antenna (in
pounds). Add the K-factor of each antenna on the mast together for your
total system K-factor. The resulting number (in foot-pounds) must be less
than the K-factor rating for the rotator.

For example, the Hygain TH-11DXS has a 12.5 square foot windload. The Yaesu
G-800DXA rotator will handle up to 21.5 square feet. But, when you multiply
the antenna's turning radius, which is 22 feet, by the weight of the
antenna, you get a K-factor of 1,936 foot-pounds. So, although the windload
rating of the rotator would seem to indicate that it will handle the
antenna, the K-factor of the antenna is greater than what the rotator will
handle (1,302 foot-pounds). If we look at the K-factor rating of the Yaesu
G-1000DXA, we see that it is 1,663 foot-pounds, so it's not even the
correct choice. At this point we would need to use a G-2800SDX, which will
handle a K-factor of up to 6,870 foot-pounds, which is obviously much
sturdier than what we really need.

I still don't think this tells the whole story because it is the inertia
that is the problem.  (The weight at a distance from the mounting point)
The formula assumes the weight is evenly distributed over the boom length.
I had a light (relatively) 4 element triband quad on a 30 foot boom.  I
could not move the mounting point so I put a weight on one end to balance
it.  It stripped the teeth out of several rotors...

Chuck W5PR

On Wed, May 4, 2016 at 7:48 AM, StellarCAT <rxdesign at ssvecnet.com> wrote:

> I believe it was called K factor.... the turning radius X the weight of
> the antenna... if more than one antenna add these individual values... it
> is indeed more a rating of the effect of the force and momentum on the
> gearing. The K factor is also an important consideration for towers and has
> its own page on the Heights Tower Systems site... Texas Towers lists (some)
> K factors of rotors – there was a site at one point listing a good share of
> rotors but I can’t seem to find it again.
>
> Gary
> K9RX
>
>
> Is there a better way to determine if a rotator can handle an antenna
> basedon its size rather than just its wind area? Lots of people here
> haverecommended the M2 Orion 2800, which on paper is rated for 35 sqft,
> butwhen I asked the tower installer who will be putting the antenna up
> whetherhe thought the Orion could handle the OB17-4, he said that antenna
> willtear an Orion up and strip the gears on the output shaft in short
> order.The OB17-4 has a 39' boom, 17 elements, of which the longest is 48',
> andweighs 220 pounds. I've noticed that some rotators don't list a sqft
> ratingat all, but give turning and braking torque in Nm. Is there a way
> tocalculate how many Nm would be required to rotate an antenna and keep it
> inplace? I do have an RT-21 controller, which has slow ramp-up/ramp-down
> toease the load on the rotator by starting and stopping it slowly.The
> various vendors aren't much help with this as they all say their rotators
> will handle the load, while at the same time saying theircompetitor's
> product won't.73, Jerry
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